Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state imaging sensor such as a CCD sensor or C-MOS sensor used in a compact image pickup device. More particularly, the present invention relates to an imaging lens which is built in an image pickup device mounted in high-functional products such as smart TV and 4K TV, information terminal devices such as game console and PC, and mobile terminal devices such as smart phone, mobile phone, and PDA (Personal Digital Assistant).
Description of the Related Art
In recent years, there has been noticed high functional products such as smart TV to which functions of PC is added, and 4K TV having resolution four times as much as full HD and pursuing high image quality. Since smart TV is becoming multi-functional in addition to high functionalization, development in the future in this field is especially expected. As one of the function, there is provided function for mounting image pickup device and taking videos and images, and communicating. By using this function, for example, various applications is expected such as high precision people meter using TV phone and facial recognition technology, furthermore, security measure and monitoring pets by mounting motion detection function, and others. Moreover, high resolution as 4K TV enables people to enjoy realistic movies which they feel from obtained images as if actual objects exist there. Such function is expected to be more popularized than usual by appearance of smart TV and so on. On the other hand, recently, the products such as digital photo frame on which camera function is mounted are sold, and it is thought that the market of camera will more expanded.
If communication is made by using TV phone, for example, information such as face expression of a speaker and situation around is important in television conference to which a plurality of people attend. Also, if faces of a human being and an animal are recognized by using facial recognition technology, it is desirable that the faces are recognized extremely precisely. The imaging lens is anticipated to provide high resolution, compactness, a wide field of view and high brightness for the imaging lens applied to devices having high resolution.
However, it is difficult for the conventional technology to fully meet such demand. In the Conventional art, as for image pickup devices incorporated into a mobile terminal device such as smart phone, imaging sensors of about 1/3.2 to ¼ inch which are appropriate for compactness of the devices are mainly used, and the number of pixels was mainly 5,000,000 to 8,000,000. Also, applied imaging lens has been required to be compactness appropriate for its size and to have resolution performance corresponding to the number of pixels of imaging sensor. On the other hand, relatively large imaging sensors of less than ½ inch are applied to image pickup devices incorporated into high-functional products such as smart TV and 4K TV which pursue much higher resolution than that of conventional full HD TV. For example, if the imaging sensor has size of 1/1.5 inch and its pixel pitch is 1.4 micron, the number of pixels will be 35,000,000, and it becomes possible to apply to the resolution performance of the product. When the conventional imaging lens is applied to such a large imaging sensor, if scaling and enlargement of the optical system are made, there is raised the problem of deterioration of aberrations accompanying the enlargement of optical system. That is, it becomes very difficult to maintain proper high optical performance which has no problem in the conventional compact imaging sensor. Moreover, recently, not only high-functional products but also mobile terminal devices are requested to have a wide field of view. Conventionally, a field of view was generally about 70 to 80 degrees, and correction of aberration has been properly carried out within the imaging angle. When wider imaging angle is provided, however, correction of aberration in peripheral area is very difficult and there will be raised problem that high optical performance is not ensured.
As the imaging lens built in products with image pickup devices, for example, the imaging lens described in Patent Literature 1 and Patent Literature 2 are known.
Patent Literature 1 discloses an imaging lens which includes, in order from an object side, a first lens having positive refractive power and a convex surface on the object side near the optical axis, a second lens having negative refractive power, a third lens having positive refractive power and a concave surface on the image side near the optical axis, a fourth lens having positive refractive power and a convex surface on the image side near the optical axis, and a fifth lens having negative refractive power near the optical axis. The imaging lens described in Patent Literature 1 is to achieve high-functionalization by providing the five constituent lenses and achieving optimization of each constituent lens.
Patent Literature 2 discloses an imaging lens in which in order from an object side, there are arranged a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, a fourth lens group having negative refractive power, a fifth lens group having positive refractive power, and a sixth lens group having negative refractive power. The imaging lens described in Patent Literature 2 that lens configuration in optical system is designed as concentric to an aperture stop, and astigmatism and coma aberrations are suppressed and a wide field of view is achieved.